As a means of sourcing the original photographer, not so much intent to identify the people in the photographs, social media can be invaluable. It is neither unethical or legally questionable, but rather a process of using modern technology to help identify people from 40-50 years ago, in the same way that DNA was used to positively identify the bits and pieces of Richard III. I'm sure if His Grace were around today he'd be mightily impressed with the means by which we commoners can undertake detective work that may just lead us to identifying people from half a century ago and, maybe, reunite them with a lost family treasurer. The possibilities are endless. I'd say scan the prints and bang up a Facebook page e.g. LOST & FOUND. See where it goes from there.